Output windows for microwave tubes



E. LAKITS Jan. 30, 1968 6 Sheets-Sheet 1 Filed Aug. 27, 1965 Jan. 30, 1968 5. LAKITS 3,366,899

OUTPUT WINDOWS FOR MICROWAVE TUBES Filed Aug. 27, 1965 6 Sheets-Sheet 2 T1527 TE30 T1540 0,8

Jan. 30, 1968 E. LAKlTS OUTPUT WINDOWS FOR MICROWAVE TUBES 6 Sheets-Sheet 3 Filed Aug. 27, 1965 I II I Jan. 30, 1968 Filed Aug. 27, 1965 VSWR 6 Sheets-Sheet 4 Jan. 30, 1968 E. LAKITS OUTPUT WINDOWS FOR MICROWAVE TUBES Filed Aug. 27. 1965 6 Sheets-Sheet 5 3200 3300 HMH z) HMSA E. LAK'ITS Jan. 30, 1968 OUTPUT WINDOWS FOR MICROWAVE TUBES s-SheeiT'G 6 Sheet Filed Aug. 27, 1965 5 Claims. (a. 333-98 ABSTRACT OF THE DISCLOSURE A gas-tight wave permeable microwave window for microwave tubes is disclosed. The window comprises a section of rectangular waveguide locally divided by a conductive partition parallel to its broad walls into at least two sections of flat waveguide each provided with a sealed alumina ceramic block window. Said dielectric blocks have the required thickness to be halfwave resonant on a frequency of f suitably selected from the passband of a microwave tube to which they are connected. Said flat waveguide sections have a width to height ratio a/b greater than 4 to provide a Wide passband between the resonant frequencies of said dielectric blocks on the adjacent parasitic TE TE transmission modes. Said dielectric block windows are spaced apart longitudinally of the wave guide by slightly less than a quarter of a guide wave length for broad handing the complex window. And the length of said partition is substantially equal to the thickness of one of said alumina ceramic blocks plus a whole multiple of half the wavelength in said rectangular Waveguide of the selected frequency f so as to make the resonance frequency of said blocks coincide with one of the resonant frequencies of the complex window in the fundamental mode TE The present invention relates to improvements in output Windows for Waveguides for very high power hyperfrequency tubes, and more particularly to thick half-wave resonant windows.

It is known that the increase of the power of hyperfrequency tubes has led to the pressurization of the associated waveguides in order to avoid bursting, and consequently to the reinforcement of dielectric windows providing the seal and effecting the transmission of energy between the vacuum enclosure of such tubes and their output waveguide. For this purpose, in order to avoid on the one hand a reduction of the size of the windows which would lead to excessive energy concentration, and on the other hand the difficulties then arising through the brazing of thick dielectric windows on the conducting walls of waveguide-s, use has been made of conducting partitions of plates perpendicular to the electric field in order to support or divide relatively thin windows. The second of these solutions has the advantage over the first that on the one hand the superficial electric field in the vicinity of the dielectric was reduced and on the other hand that the offsetting of the window elements bounded by these partitions was permitted for the purpose of reducing the stresses due to the differences in the methcients of expansion between the dielectric and conducting materials.

Technological progress made in brazing over a large surface of materials having different coefficients of expansion has since that time made it possible to utilize thick half-wave resonant dielectric windows which in consequence have a negligible coefficient of reflection at the resonance frequency. These half-wave windows, of which the naturally narrow pass band may be suitably widened by means of conventional irises, have two ad- States Patent 0 vantages over thin windows, namely better mechanical behaviour enabling them to be subjected to pressures of 15-20 atmospheres, and increased resistance to perforation under the action of bombardment by charged particles of their face which is exposed to the vacuum, but they have the disadvantage of resonance on parasitic modes coupled to the fundamental transmission mode through the slightest dissymmetry occurring in their practical construction, thus resulting in a considerable reduction of the utilizable pass band. Since in fact these parasitic resonances generally have high Q due to the weakness of the couplings, when the output frequency of the tube coincides with that of one of these resonances the fields induced in the dielectric become very considerable and the abrupt increase in the power dissipated may cause the destruction of the window by thermal shock. A known means for removing the resonance frequencies of the parasitic modes on each side of the fundamental frequency of the window consists in reducing the section of the window in relation to that of the guide with the suitable transitions, that is to say in the case of rectangular guides reducing the height of the window in relation to the narrow side of the guide, but this results in a concentration of energy entailing risks of heating and bursting of the dielectric and a reduction of the pass band due to the change of impedance of the guide with the variation of its section.

The object of the invention is to improve the utilization of thick half-wave resonant windows in order to ensure tightness and the transmission of energy over a wide band of frequencies between the vacuum enclosure of a very high power hyperfrequency tube and an output waveguide capable of being pressurized at high pressure. Another object of the invention is to simplify the impedance matching of thick half-wave resonant windows in a great bandwidth.

According to the invention, the section of the output Waveguide of a very high power hyperfrequency tube is divided by at least one partition perpendicular to the electrical field into at least two waveguide lengths each containing a window constituted by a dielectric block with half-wave resonance on a frequency suitably selected from the pass band of said tube, the section of said lengths of waveguides being such that a wide pass band exists between the resonance frequencies of said blocks on parasitic trans-mission modes, said dielectric blocks are offset longitudinally in relation to one another by a distance close to one quarter of the wavelength in the guide of the said selected frequency, and the length of the portion of said lengths of waveguide which does not contain dielectric is very close to a whole multiple of half said wavelength, so as to obtain an optimum pass band in the fundamental transmission mode of the complex window thus produced, and the characteristic dimension of the cutoff frequency of the waveguide is locally adjusted in order to obtain optimum readjustment of the pass band of the dielectric blocks between parasitic resonance frequencies and the pass band of the window in the fundamental mode. In the case of a rectangular waveguide having a ratio a/b higher than or equal to 2 between its broad dimension a and its narrow dimension b, the guide is divided by a partition parallel to its broad side into two guide lengths which are sufliciently fiat to ensure that it will be possible to use a wide pass band between two resonance frequencies on parasitic transmission modes of the dielectric blocks which they contain. The offsetting between these dielectric blocks is preferably slightly less than one quarter of the wavelength in the guide of their half-wave resonance frequency on the principal mode TE The middle partition separating the guide into two lengths must project to the outside of the ceramic blocks by at least one tenth of this wavelength in order that the weakening of the evanescent waves due to parasitic modes may be sufficient. If this condition is fulfilled, it is advantageous to give this partition the shortest possible length, that is to say the longitudinal dimension of a ceramic block plus half the wavelength in the guide of the half-wave resonance frequency of the ceramic blocks. A wide pass band is thus obtained which is for example of the order of 20%, taking as limit a standing wave ratio of the order of 1.1 to 1.2 when the dielectric blocks are composed of an alumina ceramic material. An adjustment of the broad dimension a of the waveguide at the level of the window permits offsetting the parasitic resonance frequencies of the ceramic blocks in relation to their half-wave resonance frequency on the principal mode and thus making the pass band of the blocks between parasitic resonances coincide with the pass band of the window on the fundamental mode.

The invention will be better understood from the following description of one form of construction of a window of this type which is composed of blocks of alumina ceramic material disposed in a rectangular waveguide, and from examination of the accompanying drawings, in which:

FIGURE 1 is a diagrammatical view in perspective of a window according to the invention inside a length of rectangular waveguide, partially broken away to show the arrangement thereof,

FIGURES 2 to 5 are explanatory diagrams,

FIGURE 6 is a view in longitudinal section of an example of construction of a window according to the diagram in FIGURE 1 and FIGURE 7 is a cross-section of the same window along the line VII-VII in FIGURE 6.

The length of rectangular waveguide illustrated in FIG- URE 1 and designated generally by the reference number 10 comprises four conducting walls 1114 of which the first two 11, 12 constitute its broad sides of width a and the others, 13, 14 constitute its narrow sides of width b, the ratio a/ b being higher than 2, for example equal to 2.2, as is conventional with decimetric or centrimetric waves. The window disposed in this guide comprises two blocks 21, 22 of a dielectric material, for example of alumina ceramic material, disposed on each side of a conducting partition 23 which is fixed, for example by brazing, to the narrow sides 13, 14 of the guide, parallel to its broad sides 11, 12 at half-way between the latter. The blocks 21 and 22 each occupy the entire section of half the guide in which it is placed, and they effect the sealing of said guide. Their thickness 2 in the direction of the axis of the waveguide is such that they attain halfwave resonance on a frequency f conveniently situated in the pass band of the hyperfrequency tube with which the guide 10 is associated. It is known that said thickness is given by the relation where e is the dielectric constant of the block and M the wavelength corresponding to the frequency f The length l of the middle wall 23 on each side of the series of blocks 21, 22 is sufficient to ensure that each of them can be considered as a window disposed in a very flat waveguide having a flattening ratio higher than 4, for example of the order of 4.4. It is known that in these conditions each of these windows has a pass band of the order of 20% bounded by the resonance frequencies of the block 21, 22 on the parasitic modes "IE and TE If the two blocks 21 and 22 were disposed Opposite one another, they would as a whole behave substantially like a cavity in symmetrical transmission having a narrow pass band, as regards the propagation of the fundamental mode TE For example, for blocks 21, 22 of alumina,

a cavity of this type would have a Q of the order of 3, and consequently a pass band of about 3.3%, limiting the standing wave ratio to 1.1. Its wide band matching may be effected in conventional manner by forming a symmetrical filter having the window as central cavity, but a preferred solution consists in effecting this matching by lonigtudinal offsetting d between the blocks 21 and 22 and a judicious selection of the different parameters defining the window, as will be seen from examination of FIG URES 2 to 5, which summarize the results of calculations and experiments made by taking, by way of example without limitation, blocks 21, 22 of alumina having a dielectric constant 6:9.2.

FIGURE 2 shows the displacement of parasitic resonance frequencies of an alumina window having a flattening higher than 4 in dependence on the ratio f/f between its resonance wavelength t on the mode TE and the cut-off wavelength x =2a of the guide. The quantity shown as abscissa is the ratio 7 between the frequency of the signal applied and the resonance frequency f of the window on the mode TE It is seen that the resonances on the mode TE are always obtained at frequencies lower than the cut-off frequency of the mode TE that there is a first pass band of the order of 20% etween the cut-off frequency and the resonance frequency on the mode TE and that a second pass band situated between the resonance frequencies on the modes TE and T13 is displaced ap proximately by a band contained between 1: 0.8 and f i to the hand between f f and f:l.2f when A /ZII varies from 0.6 to 0.8. It will be seen later on how this possibility of displacing the central frequency of the pass band of the dielectric blocks 21, 22 in relation to their resonance frequency on the mode TE by a local modification of the parameter a is utilized for the readjustment of the pass band of the structure.

FIGURES 3 and 4 show how it is possible to determine the values of the longitudinal offsetting d between the blocks 21, 22 and of the total length of the partition 23 in order that the structure illustrated in FIGURE 1 may have as wide a pass band as possible. The reduced variable x shown as abscissa in these figures is tied to the wavelength in the guide it by the relationship:

K) a: K I

where A is the special value assumed by M when the dielectric blocks 21, 22 resonate on the fundamental mode TE Voltage standing wave ratio VSWR is plotted as the ordinate of FIGS. 3 and 4.

Whatever the parameters selected, the structure is always matched for x=0, that is to say for the resonance frequency of the dielectric blocks 21, 22, and examination of FIGURES 3 and 4 shows that the pass band obtained by judicious selection of the parameters d and l is considerably decentred in relation to this frequency, which is situated fairly near its upper limit. Consequently, the value which is convenient according to FIGURE 2 for the parameter a /2a, on which the position of the parasitic resonances of the blocks in relation to their resonance on the fundamental mode depends, is of the order of 0.65.

The curves 1, 2, 3 in FIGURE 3 have been plotted to show the influence of the offsetting d between the blocks 21 and 22 on the pass band, the total length of the partition 23 being constant and fixed so that the length d+2l of the lengths of fiat wave-guide bounded by the partition 23, less the thickness 2 of the dielectric contained by each of them, will be equal to A /2. These curves 1, 2, and 3 corresponding to offsetting d, respectively, equal to 0.2m 0.25% and 0.3k show that the optimum value of (I is slightly less than A /4.

The curves 4, 5, 6 in FIGURE 4 intended to show the influence of the length d+2l, are plotted for a constant offsetting value between blocks of d t 4 and for values 5 of d+2l respectively equal to 0.45x 0.50% and 0.55x When the pass band is calculated on a wide range of values of x, it will be observed that there are a certain number of resonances the frequency of which depends as a first approximation only on 80 and that whenever d+2l tends towards a multiple of A /2, one of these resonances tends towards x=0, disappearing at the limit. It will moreover be observed that the interval between two of these resonances is the greater, the smaller the value of For d+2l it is therefore necessary to choose the value A /2 which gives the largest interval, eliminating one of the resonances, as shown by curve in broken lines, which has only two resonances, whereas curves 4 and 6 have three each.

FIGURE 5 shows, in solid lines plotted against frequency f (mc./s.), the calculated voltage standing wave ratio VSWR and, in broken lines, the voltage standing wave ratio measured experimentally, for a window according to FIGURE 1 provided in a standard waveguide in band S, having the dimensions 11:72.15 mm. and 12:34 mm., the thickness of the partition 23 separating the waveguide into two equal parts being 1 mm. The resonance frequency of the alumina blocks 21, 22 being equal to 2,940 rnc./s., their offsetting is 33.2 mm., and the length l=10.7 mrrr, which, taking into account the correction specified by N. Marcuvitz (Waveguide Handbook, McGraw-Hill Book Company, Inc, New York, 1951, pages 353-354), corresponds to a ratio very close to 0.5.

The curve in solid lines has been calculated for the following values of the parameters:

A 241:0.7 =9.20 d=O.23)\ and Although these values correspond only approximately to the physical dimensions of the window produced, agreement between the calculated and measured standing wave ratios is fairly satisfactory. It has been found experimentally that suppression of the resonance of the structure at the resonance frequency of the ceramic blocks on the mode TE is not very critical in dependence on the length of the partition 23 and the differences of a few tenths of a millimeter may be tolerated without disadvantage. The parasitic resonances of the alumina blocks on the modes TE and TE which appear at the frequencies resulting from FIGURE 2 for the value k /2a=0.7 adopted may be olfset by one hundred mc./ s. towards the low frequencies by a widening of the waveguide length 10, bringing x /2a to a value close to 0.65 at the level of the window, with the suitable progressive transitions.

FIGURES 6 and 7 show an example of construction of a window having the characteristics explained above and equipped in addition with a water circuit for cooling the ceramic blocks.

The actual window is contained in a cylindrical block of copper 20 brazed on two aligned elements 10, 10' of a waveguide, said copper block containing, in alignment with the guide, two openings 31, 32 of rectangular crosssection traversing it from side to side and separated by a middle partition 33. These apertures contain the alumina blocks 21, 22 previously brazed to the interior of a jacket 4-1, 42 of stamped copper, said jacket being machined to the form of a cylinder of rectangular section while its edges, and also the longitudinal edges of the blocks of alumina, are carefully rounded. Wide offset apertures 51, 52 are cut in the block 20 perpendicularly to its axis, the first level with the alumina block 21 and the second level with the alumina block 22. The apertures 51 and 52 are identical, diametrally opposite, and have sufficient depth to ensure that each of them will penetrate on one side into the thickness of the middle partition 33, their oftsetting being such that the thickness of metal necessary for the strength of the copper block 20 will be left between them in the direction of the axis of the guide. The copper block 20 is brazed in a jacket 53 which closes the apertures 51 and 52 so that each of them constitutes a sealed chamber which surrounds the jacket 41 or 42 of the alumina block 21 or 22 with the exception of the ends of the jacket which are inserted and brazed in the walls of the openings 31 and 32 on the outside of the apertures 51 and 52.

The chambers 51 and 52 communicate through the middle part of the partition 33, which middle part is not cut into, through a series of orifices 34 substantially parallel to the axis of the waveguide 10, 10. The jacket 53 is pierced by two orifices 54 and 55 enabling a circulation of water for the purpose of cooling the alumina locks 21, 22, to be established through the chambers 51, 52 and their communication orifices 34. Finally, two round rods 35, 36 of the same metal as the waveguide elements 10, 10 are brazed across said elements in the middle of their narrow sides, parallel to their broad faces, near their ends so as to provide a gradual transition between the waveguide elements 10, 10' and the partition 33 separating the alumina blocks 21 and 22.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

- I claim:

1. A thick half-wave resonant complex window for ensuring tightness and the transmission of wave power on the fundamental mode TE between the vacuum enclosure of a very high power microwave tube and a pressurized output waveguide comprising, a given length of rectangular waveguide, at least one partition parallel to the broad walls of said waveguide dividing said given length of waveguide into parallel sections of flat Waveguide each internally provided with a tightly sealed thick dielectric block, said dielectric blocks having the required thickness to be halfwave resonant on a frequency selected from the pass-band of said tube, said dielectric blocks being offset longitudinally with respect to each other along the direction of said waveguide by a distance slightly less than one quarter of the wavelength of said frequency in said Waveguide and the length of the portions of said flat waveguide sections which do not contain dielectric being substantially equal to a whole multiple of half the wavelength of said selected frequency in said waveguide, whereby a wide pass band is obtained between the resonant frequencies of said dielectric blocks on parasitic transmission modes and said complex window is matched to a corresponding wide pass band.

2. A thick half-wave resonant complex window for ensuring tightness and the transmission of wave power on the fundamental mode TE between the vacuum enclosure of a very high power microwave tube a pressurized output waveguide comprising, a given length of rectangular waveguide, at least one partition parallel to the broad walls of said waveguide dividing said given length of waveguide into parallel sections of flat waveguide each internally provided with a tighly sealed thick alumina ceramic block, each of said blocks having the required thickness to be half-wave resonant on a frequency i selected from the pass-band of said tube, each of said sections of flat waveguide having a breadth to height ratio a/b greater than 4, said blocks being offset longitudinally with respect to each other along the direction of said waveguide by a distance slightly less than one quarter of the wave length of said waveguide at the halfwave resonance frequency f the length of the portions of said flat waveguide sections which do not contain dielectric being substantially equal to a whole multiple of half the wavelength of said selected frequency in said waveguide and the breadth, a, of said sections of flat waveguide containing said dielectric blocks being between limits satisfying the condition where A is the wavelength in free space corresponding to the selected frequency f for shifting the resonance frequencies of said alumina ceramic blocks on the parasitic TEsO, TE transmission modes whereby said parasitic resonance frequencies are separated by a wide pass band, said complex window is matched to a wide pass band and the said wide pass bands are driven in optimum correspondence.

3. The apparatus according to claim 2 wherein said given length of Waveguide being divided by one partition into two parallel sections of flat waveguide, said partition has a length substantially equal to the thickness of one of said alumina ceramic blocks taken in the direction of the waveguide, plus a whole multiple of half the wave length in said rectangular waveguide of said half-wave resonance frequency so as to make the resonance frequency of said blocks coincide with one of the resonant frequencies of the complex window on the fundamental TE) mode.

4. The apparatus according to claim 3, wherein the partition projects to the outside of said alumina ceramic blocks by at least one-tenth of said wave length in the guide, whereby weakening of the evanescent waves due to said parasitic modes is obtained.

5. The apparatus according to claim 4 wherein said partition has a length substantially equal to the thickness of one of said alumina ceramic blocks, plus half the wave length in said rectangular. waveguide of said half-wave resonance frequency, whereby a maximum distance is obtained between the resonant frequencies of said complex window on the fundamental TE mode.

References Cited FOREIGN PATENTS 9/1959 Germany. 4/1964 Great Britain. 

